1. Field of the Invention
The invention pertains to the field of semiconductor optoelectronic devices. More particularly, the invention pertains to high-power high-brightness semiconductor diode lasers with a narrow beam divergence and to arrays, bars and stacks based thereupon.
2. Description of Related Art
There is a need in high-power semiconductor diode lasers for numerous application including, but not limited to welding, material processing, projection television, frequency conversion, etc. For these applications high power and high brilliance (power emitted in a unit solid angle) are of key importance.
Conventional prior art edge emitting laser have severe limitations. First, the output power is limited by the catastrophic optical mirror damage, and all technological improvements including facet passivation, zinc diffusion, or proton bombardment still have limitations in optical power density. To achieve higher power by keeping the same power density one needs using broad area lasers. However, the lasing from broad area lasers is typically multimode and also suffers from beam filamentation which renders the laser radiation not focusable.
Using semiconductor diode laser as pump source for pumping solid state laser is extremely expensive. Therefore there is a need in the art in semiconductor diode laser allowing high power narrow beam divergence single vertical mode single lateral mode lasing. The present application discloses an approach enabling the required solution.
Referring to prior art laser, the following should be noted. To ensure lasing in a single vertical mode, a laser with a narrow vertical waveguide is typically used. FIG. 1(a) shows schematically a cross-section of a prior art broad area edge-emitting laser (100) having a narrow vertical waveguide (103). By a narrow waveguide we mean a waveguide, the thickness of which does not exceed three times the wavelength of the emitted laser light in the vacuum. The narrow waveguide (103) in FIG. 1(a) is sandwiched between a bottom cladding layer (102) and a top cladding layer (109). The laser (100) is typically grown epitaxially on a substrate (101), which is preferably n-doped. Then the bottom cladding layer (102) is preferably n-doped, and the top cladding layer (109) is preferably p-doped. The waveguide (103) preferably comprises an n-doped part (104), an undoped or weakly n-doped part (105), an undoped active region (106), an undoped or weakly p-doped part (107), and a p-doped part (108). The active region comprises a double heterostructure, one or multiple quantum well, one or multiple layers of quantum wires, one or multiple layers of quantum dots, or any combination thereof. The active region generates light, when a forward bias (113) is applied. The bias is applied via a bottom n-type contact (111) mounted on a back side of the substrate (101) and a top p-type contact (112) which is preferably mounted on a heavily p-doped contact layer (110) grown on top of the top cladding layer (109). To ensure that the laser light (115) comes out from the front facet (116) and does not come out from the rear facet (117), an antireflecting coat formed from a multilayer dielectric structure is preferably deposited on the front facet (116), and a highly reflecting coat formed from a multilayer dielectric structure is preferably deposited on the rear facet (117).
FIG. 1(b) show schematically a view from the facet on the edge-emitting laser (100). A ridge stripe (118) is formed by selective etching on the top cladding layer (109), and the top contact (119) remains only on top of the ridge. To ensure a high output power, a broad area laser is typically fabricated, wherein the stripe width is about 50 micrometer or more.
Although a narrow waveguide can typically support only one vertical optical mode, a plurality of lateral optical modes can be present in the laser radiation. FIG. 2(a) shows schematically a view from the facet on the laser (100) showing schematically a fundamental lateral mode (221) and one of high-order lateral modes (222). Due to multi-mode lasing, the beam quality of the laser radiation from a broad area laser is low. In addition, filamentation can occur which even more deteriorates the beam quality and renders the laser beam not focusable.
Different approaches have been proposed to stabilize lateral modes, particularly in broad area lasers, by selective etching and overgrowth resulting in the resonant-antiguided array (so called ROW array) and the antiresonant reflecting optical waveguide (ARROW) concepts. One of such approaches is illustrated in FIG. 2(b) wherein dielectric insertions (231) in the waveguide are introduced to stabilize the lateral optical modes. FIG. 2(b) shows schematically a view from a facet on a prior art edge-emitting laser (200), in which insertions (231) of a foreign material are introduced into the waveguide. These insertions can have the refractive index either lower or higher than that of the waveguide and they serve to stabilize the lateral field of the optical mode emitted from the laser. Such approach includes complex and expensive technological steps, i.e. etching and overgrowth and in fact does not prevent multi-mode lasing.
Conventional multistripe processing, which is cost-effective, does not enable single mode operation either. Moreover, if conventional laser structures with a narrow waveguide are used, the optical field becomes localized separately under each stripe as shown in FIG. 3(a). FIG. 3(a) shows schematically a view from a facet on a prior art edge-emitting laser (300) with multiple ridges (318). Top contact is usually mounted on the ridges. The profile of the injection current in the active region depends on the particular current spreading profile. Typically, optical gain in the active region is generated underneath the stripes, and in the other parts of the active region, the active medium remains absorbing. The lateral optical modes in a waveguide narrow in the vertical direction are generated under each of the stripes, as is shown schematically (321). The lateral optical modes generated under neighboring stripes, do not overlap. This hinders the formation of a single coherent optical field and thus leads to filamentation of the emitted laser light, and no benefit in brightness or focuseability may be expected.
Thus, there exists a strong need in the art for broad area filament-free, single lateral mode lasers having a narrow lateral beam divergence. Solving the above problem is possible with the present invention.